There are many types of circuits for powering a load. One such circuit is a resonant inverter circuit, which receives a direct current (DC) signal, from a rectifier for example, and outputs an alternating current (AC) signal. Resonant inverter circuits are used in a wide variety of devices, such as lamp ballasts. The AC output can be coupled to a load, such as a fluorescent lamp, or to a rectifier so as to form a DC-DC converter.
Resonant inverter circuits can have a variety of configurations. For example, a half-bridge inverter circuit includes first and second switching elements, such as transistors, coupled in a half-bridge configuration. A full-bridge inverter circuit includes four switching elements coupled in a full-bridge configuration. Half-bridge and full-bridge inverter circuits are typically driven at a characteristic resonant frequency determined by the impedance values of the various circuit elements, including a resonant inductive element.
Conventional ballast circuits typically include an output transformer inductively coupled to the resonant inductive element for isolating lamps from the resonant circuit. The output transformer is a well known configuration for meeting applicable Underwriters Laboratories (UL) lamp ballast ground fault standards. In general, the current from the ballast lamp terminals is limited to a predetermined level with respect to ground. By limiting the current, a person touching the lamp terminal so as to form a path to ground through the person's body is not electrocuted.
FIG. 1 shows a typical prior art ballast circuit 10 having a conventional output isolation transformer 12. A rectifier/filter 14 receives an AC input signal on first and second input terminals 16a,b and provides positive and negative voltage rails 18, 20. Inductively coupled inductors L1-A, L1-B can be provided on the respective positive and negative rails 18, 20. First and second switching elements 22, 24 are coupled across the rails in a well known half-bridge configuration. A primary winding 26, e.g., 1.5 mH 50 turns, of the output isolation transformer combines with a resonating capacitor 28 to form a parallel resonating circuit. A secondary winding 30, e.g., 100 turns, of the transformer energizes first and second lamps LP1, LP2 each of which is coupled in series with respective lamp capacitors CL1, CL2. In this well known configuration, the secondary winding 30 of the transformer isolates the lamp terminals from the resonating circuit so as to limit the ground fault current flow. In the event a technician inadvertently touches a lamp terminal and thereby provides a current path to ground, the current flow through the technician's body is limited to a safe level to prevent injury. Underwriter's Laboratories promulgates standards for acceptable ballast ground fault current levels.
While the output isolation transformer provides safety, it is relatively bulky so as to require significant space on the ballast circuit board. The output transformer also consumes a relatively high amount of power. In addition, the transformer performance is negatively impacted in some applications by the corona effect. For example, in so-called instant start ballasts, in which a relatively high voltage, e.g., 500 VRMS, is applied to the lamp terminals to initiate current flow through the lamp, the transformer must provide this voltage to strike the lamp. Such a voltage can cause the transformer operating characteristics to degrade over time.
It would, therefore, be desirable to provide a ballast circuit having an enhanced output isolation configuration.